Internal combustion engines commonly have kill-switches incorporated into their ignition systems so that manual or automatic activation of the kill-switch results in an immediate shut down of the engine. Various types of kill-switches and accompanying circuitry have been utilized across a variety of applications, including light-duty combustion engines. The term ‘light-duty combustion engine’ used herein broadly includes all types of non-automotive combustion engines, including two- and four-stroke engines used with hand-held power tools, lawn and garden equipment, lawnmowers, weed trimmers, edgers, chain saws, snowblowers, personal watercraft, boats, snowmobiles, motorcycles, all-terrain-vehicles, etc.
Under certain operating conditions, activation of some kill-switches can result in an engine backfiring or emitting a flame from an exhaust pipe outlet. This is because many engines currently operate on leaner air/fuel mixtures or utilize catalytic converters in order to improve their emission characteristics. This can result in the engine having ‘hot spots’, which generally include any high temperature area of the engine capable of causing an unintended combustion of air/fuel charges. Hot spots are commonly located in the combustion chamber and exhaust system and can unintentionally ignite air/fuel charges, thereby causing the engine to backfire or to emit a flame. One factor that can contribute to air/fuel charge build up, and hence unintentional combustion, is when an engine operating at high speeds is suddenly shut off via activation of a kill-switch.
For example, a typical hand-held trimmer operating at an engine speed of 10,000 rpm can take approximately 2.5 sec to come to a complete stop after the kill-switch has been activated and the ignition system has stopped sending an ignition pulse to the spark plug. During that time, a considerable amount of air/fuel mixture passes through the combustion chamber and enters the exhaust system, at which point it can build up and be ignited by one or more hot spots. As previously mentioned, such an ignition can result in the engine backfiring, producing a loud noise commonly called an “after boom”, and/or discharging a blue flame.
Some applications address this issue by using a solenoid that shuts off fuel in response to kill-switch activation. For example, a typical lawn tractor operating at an engine speed of 3,600 rpm can take five seconds or so to come to a complete stop after a kill-switch has been activated. During this time, the solenoid blocks excess fuel from entering the muffler and thereby prevents backfiring.